Extended Abstract (1.3M)J1.7
Ocean-atmosphere interaction within equatorially trapped atmospheric waves
George N. Kiladis, NOAA/AL, Boulder, CO; and K. H. Straub
It is well documented that the Madden-Julian Oscillation (MJO) is associated with variations in sea surface temperature (SST) over its domain, the warm pool of the eastern Indian and western Pacific Oceans. The local relationship is such that elevated SST precedes the disturbance, with suppressed SST following the passage of the eastward propagating convective envelope. It is generally thought that fluxes of radiation along with low wind speed are responsible for increasing the SST prior to the MJO, with mixing and radiation responsible for cool SST its wake. The time scale necessary for this local forcing is consistent the relatively slow 5 m/s phase speed of the MJO. The MJO can also alter remote SST due to its effect on the easterly trade wind regime over the Pacific sector. The relatively slow time scale of the MJO enables the associated fluctuations to be studied using satellite and in situ derived SST operational products. There exists a host of other equatorially-trapped atmospheric waves that have the potential to alter equatorial SST, but on a much higher frequency. The local wind and SST signals associated with these disturbances can be detected through the use of TAO/TRITON array data. In particular, convectively coupled Kelvin waves, which propagate eastward at phase speeds around 15 m/s, have been shown to be associated with local SST forcing. A case study of such an interaction during May-June 1998 is examined in detail. In the months prior to this period, heat content over the tropical Pacific slowly decreased as a result of oceanic wave dynamics, although the SST remained quite high. Then in May equatorial Pacific SST plummeted by up to 8C, bringing a dramatic end to the strong 1997-98 El Nino conditions. We show that this SST tendency was primarily related to the rapid onset of oceanic upwelling induced by the sudden increase in trade wind flow associated with the MJO/Kelvin wave complex. In late April 1998, a convectively coupled Kelvin wave signal propagated from the Atlantic, through Africa, and into the Indian Ocean where it appears to have triggered an MJO. As the MJO developed, surface winds over the tropical Pacific went from westerly to easterly. At the same time the Kelvin wave continued rapidly across the Pacific in the convective and dynamical fields. The combination of wind forcing from the MJO and Kelvin disturbances resulted in the upwelling of cold subsurface water in the central and eastern Pacific, decreasing the SST dramatically. Once the SST decreased to levels unable to support convection, the convective field was re-established farther west in its more usual position, and the 1997-98 warm event was terminated. The potential role of such waves in the demise of the 1982-83 and 1991-92 warm events will also be discussed. In addition, the Kelvin pulse of 1998 is shown to have short-lived response of cooling SSTs following the passage of its convective envelope, most likely due to the combination of precipitation and radiative forcing.
Joint Session 1, ENSO and Global-scale atmosphere-ocean coupling (Joint with the 14th Symposium on Global Change & Climate Variations and the 12th Conference on Interactions of the Sea and Atmosphere )
Thursday, 13 February 2003, 3:30 PM-5:45 PM
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